The present invention relates generally to voltage regulators or control systems employing triacs to control the power supplied to a load from an alternating current (a.c.) voltage source.
As is known, a triac is a bidirectional semiconductor device which exhibits the property of being "gated" into conduction upon the simultaneous application of a voltage between its two main electrodes and a gating signal applied to a third or gate electrode. Triacs are normally rendered nonconductive by removing the voltage from the main electrodes; for example, by the source voltage crossing the zero axis.
In those instances where the a.c. source voltage is "clean", that is the voltage follows a substantially pure sinusoidal wave shape, few problems are experienced in maintaining good control. Where, however, the source voltage is "dirty" and includes any significant number of transients, more serious problems are presented. If a transient of a sizeable magnitude in a direction to reduce source voltage occurs at the same time that a pulse gating signal is applied to the triac, the triac may not fire (become conductive) and control may be lost. In a similar manner, if a transient of this nature occurs shortly after the triac begins conduction but prior to the establishment of a sizeable current in the triac, the current may extinguish and once again control would be lost. The loss of control can be particularly troublesome when that being supplied is a transformer coupled load. In this case, with the cessation of the triac conduction, buildup of a d.c. current in the transformer may develop with subsequent loss of control resulting in transformer saturation which, in turn, could cause heavy line currents.
One prior art method of overcoming this problem is to use what is known as pulse train firing. In its simplest conceptual form, pulse train firing entails the sensing of a point in the a.c. source wave form where initiation of conduction is desired. This sensing must be accurate and employed to gate, for a specified period within the half cycle of the source voltage, a series of pulses from an oscillator to the gate electrode of the triac. Thus, if the device should fail to conduct due to a transient "spike" in the source voltage, as soon as the transient disappears the device will be gated into conduction by the next pulse of the train. While prior art pulse train gating is a very satisfactory solution from an operational standpoint, it is also one which is relatively expensive which makes its use difficult to justify in many instances. The expense of the pulse train firing system results primarily from the fact that the point of conduction initiation must be accurately determined which requires, in the usual case, that the normal zero crossover point of the source voltage be accurately determined. To do this, the customary practice is to provide an accurate d.c. supply voltage to energize a plurality of integrators which are used to cancel out the transient effects and provide the zero crossing detection.